Set of three level concurrent word line bias conditions for a NOR type flash memory array

ABSTRACT

In the present invention a method is shown that uses three concurrent word line voltages in memory cell operations of an a NOR type EEPROM flash memory array. A first concurrent word line voltage controls the operation on a selected word line within a selected memory block. The second concurrent word line voltage inhibits cells on non selected word lines in the selected memory block, and the third concurrent word line voltage inhibits non-selected cells in non-selected blocks from disturb conditions. In addition the three consecutive word line voltages allow a block to be erased, pages within the block to be verified as erased, and pages within the block to be inhibited from further erasure. The three consecutive voltages also allow for the detection of over erasure of cells, correction on a page basis, and verification that the threshold voltage of the corrected cells are above an over erase value but below an erased value. The methods described herein produce a cell threshold voltage that has a narrow voltage distribution.

This is a division of patent application Ser. No. 09/978,230, filingdate Oct. 16, 2001, now U.S. Pat. No. 6,620,682 Novel Set Of Three LevelConcurrent Word Line Bias Conditions For A Nor Type Flash Memory Array,assigned to the same assignee as the present invention, which claimspriority to Provisional Patent Application serial No. 60/271,644, filedon Feb. 27, 2001, herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor memories and inparticular a three level concurrent word line bias condition for NORtype flash memory arrays.

2. Description of the Related Art

In today's flash EEPROM technology, a plurality of one-transistor EEPROMcells has been configured into either NAND-type or NOR-type memoryarrays. For the NAND type cell array, the sources and drains of theflash cells are connected in series to save die size for the reason ofcost reduction. In contrast, for NOR-type cell array, the drains andsources of the cells are connected in parallel to bit lines and sourcelines, respectively, to achieve high read speed at sacrifice of theincrease in die size. It is well known that the NAND-type cell arraysuffers no over-erase problem due to its unique array structure allowingno leakage path during read. For a one transistor (non-split gate)NOR-type cell array, the over erase problem may or may not occur, andthe over erase problem is subject to the choice of erase and programmethods. Conventionally, a program operation is performed on the basisof bit-by-bit method but erase is performed collectively on all cells ina block. In both the NOR-type or NAND-type flash memory, the entireflash chip is divided into several blocks, and typically, the size ofeach flash block ranges from 64 Kbits to 512 Kbits. An erase operationis performed prior to program operation, and in a NAND-type flashmemory, the erase is performed on a block (sector) basis and program isperformed on a page basis.

A page is usually defined as a word line and a block is defined as manyword lines which share common bit lines within the same divided block.Although several methods of erase and program operations have beenproposed, in the current NAND type flash memory, the definition of eraseand program operations is unified. The erase operation is to decreasethe Vt (threshold voltage) of the cells that are physically connected tothe same erased word line or the word lines in the same block. Incontrast, the program operation is to increase the Vt of cells ofselected erased word line or word lines in the selected block. Thenon-selected cells in the non-selected word lines in the selected blockor the non-selected blocks remain undisturbed.

The following U.S. patents of prior art are directed toward the detaileddescription of NAND type flash EPROM's.

A) U.S. Pat. No. 6,038,170 (Shiba) is directed toward a nonvolatilememory of a hierarchical bit line structure having hierarchical bitlines which includes, a plurality of sub-bit lines.

B) U.S. Pat. No. 5,464,998 (Hayakawa et al.) is directed toward anon-volatile semiconductor memory device including NAND type memorycells arranged in a matrix pattern over a semiconductor substrate.

Up to the present, the definition of erase and program operations for aone-transistor cell, NOR-type flash memory is inconsistent. Erase couldbe defined to increase cell's Vt and program to decrease cell's Vt orvise versa depending on the preferred flash technology and its designtechniques. The following is a summary of erase and program operationsfor state of the art one-transistor (non-split-gate) NOR-type flashEEPROM technologies.

I) FN (Fowler-Nordheim) Block erase, CHE (channel hot electron) program,one-transistor cell, NOR-type flash ,EEPROM technology. The typicalexample is an ETOX flash cell. In this prior art, programming isperformed on bit-by-bit basis to increase the Vt of the cells by usingthe CHE method while erase is performed on block basis to decrease theVt of the cells by using FN-tunneling method. The CHE program consumesmore than 300 uA per bit, therefore only a few bits can be programmed ata time by an on-chip charge pump having an economic semiconductor area.Unlike CHE, FN-tunneling erase requires only 10 nA per flash cell sothat a big block size of 512 Kb can be erased simultaneously. For a Vddvoltage of 3V or lower, about 4 bits of ETOX cells are programmed instate-of-the art design. In a CHE operation, hot electrons are injectedinto cell's floating gate with an increase in Vt. In contrast, in the FNtunneling operation, the electrons are extracted out of the floatinggate with a decrease in Vt . The erase operation is called an edge eraseoperation which is done at edge of the thin tunnel oxide between thefloating gate and the source junction. In the ETOX flash cell, thesource junction of N+ is used for an erase operation only which is madeto be much deeper than the drain node. The source junction of N+ issurrounded with lightly doped N-implant to reduce the peak electricalfield generated during erase operation at the tunneling edge. The drainjunction is formed with a shallow N+, with a P+ implanted underneath toenhance the electrical field for CHE program. The ETOX cell is madenon-symmetrical with respect to source and drain junctions of the cellin terms of cell structure and operating conditions; therefore it isvery difficult to shrink the cell using technology below 0.18 um forUltra-high integrated memory.

The key operating conditions for the ETOX technology with a cell made ona P-substrate are as follows:

Source Gate Drain Bulk a) Erase (edge) +5 V −10 V Floating OV b) Program(channel) OV +10 V +5 V OV c) Read OV Vdd + δV   1 V OV Erase Program d)Current per cell 10 nA >300 uA

The drawbacks of the ETOX flash cell are: a) a low cell scalabilityresulting from an asymmetrical cell structure with a deep sourcejunction; b) a high program current caused by the CHE program scheme; c)a high erase current resulting from using an edge-FN scheme with largesubstrate leakage current; d) severe over erase potential caused bydecreasing the Vt of cells during erase operation; e) a channel punchthrough problem in short channel lengths due to the edge erase.

The following U.S. patents of prior art are directed toward the detaileddescription of ETOX flash cell operations:

A) U.S. Pat. No. 5,712,815 (Colin et al.) is directed toward an improvedprogramming structure for performing a program operation in an array ofmultiple bits-per-cell flash EEPROM memory cells is provided.

B) U.S. Pat. No. 5,790,456 (Haddad) is directed toward an improvedmethod for performing channel hot-carrier programming in an array ofmultiple bits-per-cell Flash EEPROM memory cells in a NOR memoryarchitecture so as to eliminate program disturb during a programmingoperation.

C) U.S. Pat. No. 6,011,715 (Pasotti et al.) is directed toward aprogramming method for a nonvolatile memory which includes several stepsthat are repeated until a final threshold value is obtained.

D) U.S. Pat. No. 5,825,689 (Wakita) is directed toward a nonvolatilesemiconductor memory device including a memory cell array in which thethreshold voltage of a transistor constituting the memory cell is atground potential or less, and the source voltage condition is changed bya source potential setting circuit in accordance with a detection resultfrom a data detecting circuit.

II) AND one-transistor cell, NOR-type flash EEPROM technology. UnlikeETOX technology, in the AND one transistor prior art the program isperformed on bit-by-bit basis to decrease the Vt of cells while erase isperformed on block basis to increase the Vt of cells. Both erase andprogram operations use the FN-tunneling method which consumes only about10 nA per bit; therefore, a large number of flash cells within a largeblock can be erased simultaneously by an on-chip charge pump whichutilizes a small area on the chip. For a single low power supply, Vdd,is at 3V or below, and as many as 16 Kb of cells of the AND technologyin a block can be erased. In the AND prior art, the erase operation iscarried out by FN block channel erase, and the program operation iscarried out by page FN edge program. The edge program is at the drainedge formed by a buried N+ bit line. The electrons are injected intocell's floating gate by block channel erase operation with an increasein the Vt of the erased cells. In contrast, electrons are extracted outof a floating gate by a page edge program operation where the Vt of theprogrammed cells decreases. In this AND flash cell, the N+ drainjunction is used for program operation only and is made to be muchdeeper than the source node. The N+ drain junction is surrounded with alightly-doped N-implant to reduce the peak electrical field that isgenerated during the drain-edge-program operation. The source junctionis formed with a shallow N+ having a P+ implant underneath to preventvoltage punch-through in a short channel region during an edge programoperation. The AND cell like the ETOX cell is made non-symmetrical withrespect to the source and drain junctions in terms of cell structure andoperating conditions. Therefore, it is very difficult to shrink the ANDcell below 0.18 um technology for an ultra-high integrated memory.

The key operating conditions for this technology with cell made onP-substrate are summarized as follows.

Source Gate Drain Bulk a) Erase (channel) +OV +15 V OV OV b) Program(drain edge) +5 V −10 V Floating OV c) Read   OV Vdd 1 V OV EraseProgram d) Current per cell 10 pA 10 nA

The drawbacks of the AND flash cell are: a) low cell scalability causedby asymmetrical cell structure with a deeper drain than source junction;b) high program current resulting from the edge-FN program scheme withlarge substrate leakage current; c) severe channel punch-through problemin shorter channel length resulting from the edge program.

The detailed description of AND flash cell operations can be referred tothe following U.S. patents of prior art:

A) U.S. Pat. No. 6,072,722 (Hirano) is directed toward programming anderasing a non-volatile semiconductor storage device.

B) U.S. Pat. No. 6,101,123 (Kato et al.) is directed toward programmingand erasing verification of a non-volatile semiconductor memory.

C) U.S. Pat. No. 6,009,016 (Ishii et al.) is directed toward anonvolatile semiconductor memory which recovers variation in thethreshold of a memory cell due to disturbance related to a word line.

D) U.S. Pat. No. 5,982,668 (Ishii et al.) is directed toward anonvolatile semiconductor memory which recovers variation in thethreshold of a memory cell due to disturbance related to a word line.The nonvolatile memory continuously performs many writing operationswithout carrying out single-sector erasing after each writing operation.

E) U.S. Pat. No. 5,959,882 (Yoshida et al.) is directed toward anonvolatile semiconductor memory device with a plurality of thresholdvoltages set so as to store multi-valued information in one memory cellentitled.

F) U.S. Pat. No. 5,892,713 (Jyouno et al.) is directed toward aconfiguration that provides a nonvolatile semiconductor memory devicewhich allows high-speed block reading.

G) U.S. Pat. No. 5,757,699 (Takeshima et al.) is directed toward theprogramming of a selected memory cell which is repeated until theprogrammed threshold voltage is not greater than a predeterminedthreshold voltage.

III) FN-erase, FN-program, Metal-bit line, One-transistor, NOR-typeFlash EEPROM. Like AND flash technology, in this prior art, the programoperation is performed on a bit-by-bit basis to decrease the Vt of cellswhile erase is performed on a block basis to increase the Vt of cells.Both erase and program operations use the FN-tunneling method, whichconsumes only about 10 nA per bit without taking the greater substratecurrent into account. Therefore a large number of flash cells within abig block can be erased at one time by an on-chip charge pump havingeconomic area. For a single low power supply, Vdd, of 3V or below, alarger number of flash cells in a block can be programmed and erasedsimultaneously. In the prior art, the erase operation is carried out byFN channel-erase, and the program operation is carried out by FNedge-program. The edge-program is at the drain edge but the cellstructure is formed by a non-buried N+ bit line and a source line. Thebit line is a vertical metal line which connects all drains of the cellsin the same block for high read speed. The source lines are tiedtogether by an N+ active line, which runs in parallel to the word lines.Each source line is shared by one pair of word lines as in the ETOXflash cell array. As disclosed in the prior art, the electrons areremoved from the floating gate of the cells by drain edge FN programmingin which the Vt is decrease. Conversely, the electrons are injected intothe floating gate by channel erasing where Vt is increased. The N+ drainjunction is used for the FN program operation and is made to be muchdeeper than source node, and is surrounded with a lightly dopedN-implant to reduce the peak electrical field generated during drainedge program operation. The source junction is formed with shallow N+with a P+ implant underneath the source to prevent voltage punch-throughin a short channel region during edge-program operation. The flash cellof prior art is made asymmetrical with respect to source and drainjunctions in terms of cell structure and operating conditions;therefore, it is difficult to further shrink the memory cell forUltra-high density memory below 0.18 um technology.

The key operating conditions for the NOR type flash technology with acell formed on a P-substrate are as follows:

Source Gate Drain Bulk a) Erase (channel) +OV +15 V OV OV b) Program(drain-edge) Floating −10 V +5 V OV c) Read   OV Vdd   1 V OV EraseProgram d) Current per cell 10 pA 10 nA

The drawbacks of the NOR type flash cell are: a) Low cell scalability asa result of an asymmetrical cell structure with the drain junctiondeeper than the source junction; b) high program current caused by theedge-FN program scheme with a large substrate leakage current.; c)severe channel punch-through problem in shorter channel lengths causedby the edge program. The detailed description of the NOR type flashtechnology can be referred to in U.S. Pat. No. 5,708,600 (Hakozaki etal.) which is directed toward a method for writing a multiple value intoa nonvolatile memory capable, of multiple value data being written intoa floating gate type memory cell.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a three levelconcurrent word line bias condition and method using CHE program and FNblock erase for a semiconductor nonvolatile device and in particular,for an ETOX one transistor cell, and a NOR-type EEPROM memory arrayformed on P-substrate.

Another object of the present invention is to provide a three level wordline bias condition and methods using FN schemes for both program anderase operations for a one transistor cell, NOR type AND EEPROM memoryarray formed on P-substrate.

Another objective of the present invention is to provide a three levelword line bias condition and methods using CHE program and FNblock-erase for a semiconductor nonvolatile device, in particular anETOX one transistor cell, NOR type EEPROM memory array formed on p-wellwhich is within a deep N-well on top of p-substrate.

Still another objective of the present invention is to provide a newoperation method that employs the three level word line bias conditionto perform the bit-by-bit verify and correction for achieving both tight“0” and “1” for distributions of Vt.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described with reference to the accompanyingdrawings, wherein:

FIG. 1 is a sectional view of an ETOX type memory cell of prior art witha p+ implant and a lightly doped n− implant,

FIG. 2 is a sectional view of an ETOX type memory cell of prior art witha p+ implant,

FIG. 3 is a sectional view of an AND type memory cell of prior art witha p+ implant and a lightly doped n− implant,

FIG. 4 is a sectional view of an NAND type memory cell of prior art witha shallow source and drain,

FIG. 5 is a sectional view of an ETOX type memory cell of prior art witha p+ implant on a p-well within a deep n-well on p-substrate,

FIGS. 6a through 6 e show a single cell operating conditions of thepresent invention for an ETOX NOR flash cell array on a P-substrate,

FIGS. 7a through 7 d show additional single cell operating conditions ofthe present invention for an ETOX NOR flash cell array on a P-substrate,

FIG. 8 shows the ETOX NOR flash cell array of prior art,

FIG. 9 illustrates the bias conditions for block erase for the ETOX NORflash cell array of the present invention,

FIG. 10 illustrates the bias conditions for block erase verify for theETOX NOR flash cell array of the present invention,

FIG. 11 illustrates the bias conditions for erase inhibit for the ETOXNOR type flash cell array of the present invention,

FIG. 12 illustrates the bias conditions for correction operations forthe ETOX NOR type flash cell array of the present invention,

FIG. 13 illustrates the bias conditions for correction verify operationfor the ETOX NOR type flash cell array of the present invention,

FIG. 14 illustrates the bias conditions for program operations for theETOX NOR type flash cell array of the present invention,

FIG. 15 illustrates the bias conditions for program verify operationsfor the ETOX NOR type flash cell array of the present invention,

FIGS. 16a through 16 e show a single cell operating conditions for ANDarrays on P-substrate for the present invention,

FIGS. 17a through 17 f show additional single cell operating conditionsfor AND arrays on P-substrate for the present invention,

FIG. 18 shows an AND flash cell array of prior art,

FIG. 19 illustrates the bias conditions for random page erase operationof the present invention for the AND flash cell array of the presentinvention,

FIG. 20 illustrates the bias conditions for random page erase verifyoperation of the present invention for the AND flash cell array of thepresent invention,

FIG. 21 illustrates the bias conditions for block erase operation of thepresent invention for the AND flash cell array of the present invention,

FIG. 22 illustrates the bias conditions for block erase verify operationof the present invention for the AND flash cell array of the presentinvention,

FIG. 23 illustrates the bias conditions for block erase inhibitoperation of the present invention for the AND flash cell array of thepresent invention,

FIG. 24 illustrates the bias conditions for correction operation of thepresent invention for the AND flash cell array of the present invention,

FIG. 25 illustrates the bias conditions for correction verify operationof the present invention for the AND flash cell array of the presentinvention,

FIG. 26 illustrates the bias conditions for random page programoperation of the present invention for the AND flash cell array of thepresent invention,

FIG. 27 illustrates the bias conditions for random page program verifyoperation of the present invention for the AND flash cell array of thepresent invention

FIGS. 28a through 28 e show a single cell operating conditions for ETOXNOR arrays on a P-well for the present invention,

FIGS. 29a through 29 d show additional single cell operating conditionsfor ETOX NOR arrays on a P-well for the present invention,

FIG. 30 illustrates an ETOX NOR flash cell array on a P-well of priorart,

FIG. 31 illustrates the bias conditions for block erase operations ofthe present invention for the ETOX NOR array of prior art on a P-well,

FIG. 32 illustrates the bias conditions for block erase verifyoperations of the present invention for the ETOX NOR array of prior arton a P-well,

FIG. 33 illustrates the bias conditions for erase inhibit operations ofthe present invention for the ETOX NOR array of prior art on a P-well,

FIG. 34 illustrates the bias conditions for correction operations of thepresent invention for the ETOX NOR array of prior art on a P-well,

FIG. 35 illustrates the bias conditions for correction verify operationsof the present invention for the ETOX NOR array of prior art on aP-well,

FIG. 36 illustrates the bias conditions for program operations of thepresent invention for the ETOX NOR array of prior art on a P-well,

FIG. 37 illustrates the bias conditions for program verify operations ofthe present invention for the ETOX NOR array of prior art on a P-well,

FIG. 38a shows the Vt distribution obtained after application of theblock erase and page program sequence of operations in an ETOX NOR arrayof prior art,

FIG. 38b shows the Vt distribution obtained after application of theblock erase and page program sequence of operations of the presentinvention to the cells in an ETOX NOR array,

FIG. 39 is a flow diagram of the present invention for block eraseoperations in an ETOX NOR array,

FIG. 40 is a flow diagram of the present invention for correctionoperations of an ETOX NOR array,

FIG. 41a shows a Vt distribution obtained after application of blockerase and page program sequence of operations for a first AND cell arrayof prior art,

FIG. 41b shows a Vt distribution obtained after application block eraseand page program sequence of operations for a second AND cell array ofprior art,

FIG. 41c shows a Vt distribution obtained after application of blockerase and page program sequence of operations of the present inventionto an AND cell array,

FIG. 42a is a flow diagram block erase and page program sequence ofoperations of prior art applied to a first AND array of prior art,

FIG. 42b is a flow diagram block erase and page program sequence ofoperations of prior art applied to a second AND array of prior art,

FIG. 43 is a flow diagram of the present invention for block eraseoperations for an AND array of the present invention, and

FIG. 44 is a flow diagram of the present invention for correctionoperations for an AND array of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The operating principles of a one-transistor flash cell of the presentinvention are described with reference to FIGS. 1-5. The term “writeoperation” will be frequently used in this description and is defined asan operation, comprising erase and program operations. In one completewrite operation, erase is usually performed first on a block basisfollowed by a program operation on a page basis.

FIG. 1 shows the cross sectional view of a device structure of prior artof an ETOX flash memory cell with an n+ source 33, n+ drain 22, controlgate 10 and floating gate 11. The tunnel oxide layer 15 is formedbetween floating gate 11 and P-substrate 16. The arrow 48 shows the flowof electrons from floating gate 11 to source 33 during an edge eraseoperation. The arrow 49 shows electrons moving from drain 44 to thefloating gate 11 during CHE (channel hot electron) program operation.

Continuing to refer to FIG. 1, since the n+ source node 33 experiencesmuch higher electric field during an edge erase operation than the drain22 during CHE program operation, the source junction is made much deeperthan drain. The source 33 is lightly doped by an n− implant 34 to avoidjunction breakdown in the erase operation. The p+ implant 44 is used toincrease the substrate concentration underneath n+ drain 22 so that theCHE program operation can be achieved. An n− implant 34 is formedunderneath n+ source 33 so that breakdown can be avoided during FN edgeerase 48. A second prior art of an ETOX cell is shown in FIG. 2. Shownin FIG. 2 is a cross sectional view of ETOX cell which uses a CHEprogram 49 to increase the threshold voltage, Vt, and an FN channelerase 38 to decrease Vt. In FIG. 2, the n− implant layer is no longerrequired since channel erase 38 does not exert the high tunnelingelectric field to source junction 13.

In FIG. 3 is shown a cross sectional view an AND flash cell of prior artwith buried N+ source 13 and drain 12 which uses FN channel erase 18 toincrease Vt and FN edge program 19 to decrease Vt. Edge program isperformed similarly to the ETOX cell in FIG. 1. A lightly doped n− layer20 is required underneath n+ drain 12 to reduce the high tunnelingelectric field during program operation. The cell of FIG. 3 is anasymmetric cell and has a lower scalability as with the cell in FIG. 2.

The prior art of FIG. 4 shows a cross sectional view of a NAND likeflash cell with an n+ source 13 and drain 22. This flash cell uses FNchannel-program 28 to increase Vt and FN channel-erase 27 to decreaseVt. Unlike the previous cells, neither n− nor p+ are required for drain22 and source node 13, respectively. The cell of FIG. 4 is a symmetricalcell and has higher scalability than the cells in the cells shown inFIGS. 1, 2 and 3.

The prior art of FIG. 5 shows a cross sectional view of an ETOX cellwhich uses a CHE program 49 to increase Vt and a FN channel erase 48 todecrease Vt. The cell is formed on p-well 40 within an n-well 41 on aP-substrate 16. The n− implant layer is no longer required since channelerasing 48 does not exert the high tunneling electric field on thesource junction 13. For lower voltage operation of this cell thevoltages applied to the control gate 10 and p-well 40 can be DC-shifteddown.

The first embodiment of the present invention will be described withreference to FIG. 6 through FIG. 15. The cell used is an ETOX cell onP-substrate. FIG. 6a shows a flash cell of this invention with nodes ofD, G and S. The P-substrate is not shown and is held at ground. FIG. 6bshows a flash cell with bias conditions of this invention illustratingtwo types of ERASE operations. The first operation has D, G and S nodescoupled with floating, −10V and +5V, respectively, for edge-erase. Thesecond operation has D, G and S nodes coupled with 0V, −15V and 0V,respectively, for a channel erase, where the −15V is an exemplary value.The exact value and time of the control gate voltage is subject todifferent flash technologies. The gate voltage of −15V, and source anddrain voltage of 0V will result in a tunneling electric field in channelregion of the cell. The tunneling electric field will transportelectrons from the floating gate to the P-substrate in order to decreasethe Vt of the cells (off-state) after a predetermined erase time. Theerase operation can be performed on the basis of single-page (wordline), block (N word lines), multiple blocks (M blocks) or chip (allblocks), where typically N and M are larger than 2.

FIG. 6c shows a flash cell with bias conditions of the present inventionillustrating two types of an erase inhibit operation. The first eraseinhibit is with D, G and S nodes set to floating, −10V, +5Vrespectively, and the second erase inhibit with D, G and S nodes set to0V, 0V and 0V, respectively. This operation is intended to prevent abuild-up of disturbance to those non-selected erased cells (in eitherselected or non-selected blocks) and to achieve better endurance (numberof program and erase cycles).

FIG. 6d shows a flash cell with bias conditions of the present inventionthat illustrates the correction operation with D, G and S nodes coupledto +5V, Vcorrection and 0V, respectively. Here Vcorrection is anexemplary value for better understanding of the present invention. Theexact value and time for the control gate voltage in this operationvaries with different flash technologies. The correction operation is asoft program CHE operation. The difference between program andcorrection lies in the control gate voltage. Normal program operationhas about +10V applied to control gate and is intended to increase Vtmore than +5V. Correction has lower control gate voltage to avoid overprogram. It is used to correct the Vt of cells back to around +1V fromeither negative or below +0.5V to avoid sub-threshold leakage duringsubsequent read or program operations. The operation of FIG. 6d issometimes referred to as recovery. The data becomes “0” after thisoperation.

FIG. 6e shows a flash ETOX cell with the bias conditions of the presentinvention illustrating the CHE program operation with D, G and S nodescoupled to +5V, Vpgm and 0V, respectively. In the CHE program, there isa conduction current flowing from drain to source and causing anelectron-hole pair generated at drain node. Electrons are attracted tothe floating gate to increase Vt by the positive high voltage Vpgm,which increases the Vt of the cells. The CHE program typically consumesmore than 300 uA per cell. With weak on chip charge pump circuitsoperating at Vdd below 3V, only about 4 bits can be programmedsimultaneously. The cell data becomes “1” after this operation isperformed.

In FIG. 7a, a flash cell with bias conditions of the present inventionillustrates the program verify operation with D, G and S nodes coupledto +1V, Vpgmvfy and 0V, respectively. The Vrpgmvfy is an adjustablevoltage input to the control gate of the cells to meet different Vtrequirements in the program operation. For example, for storage of morethan 2-bits per cell, Vpgmvfy may vary from as low as 1V up to about 5Vor more. For a binary program, Vpgmvfy is set to be around +5V. Forstorage of multiple states such as 1V, 2V, 3V, and 4V, Vpgmvfy is set to1V, 2V, 3V and 4V accordingly to verify each state

In FIG. 7b, a flash cell with bias conditions of the present inventionillustrates the correction verify operation with D, G and S nodescoupled to +1V, Vcorvfy and 0V, respectively. The Vcorvfy is anadjustable voltage input to the control gate to meet different Vtrequirements in this operation. The Vt of over erased cells will berecovered back to a Vt window of between +0.5V and 1.0V, after thisoperation is performed.

FIG. 7c shows a flash cell with bias conditions of the present inventionillustrating a read operation with D, G and S nodes coupled to +1V,Vread and 0V, respectively. The Vread is an adjustable voltage input forthe control gate for the read operation. Vread can be simply set to Vdd;however, in some designs, Vread is set to a clamped value so that theread voltage applied to the control gate can be independent of Vddvariation. in some designs, the Vread voltage is boosted to be higherthan Vdd, resulting in higher read current.

Shown in FIG. 7d is a flash cell with bias conditions of the presentinvention illustrating an erase verify operation with D, G and S nodescoupled to +1V, Versvfy and 0V, respectively. The Versvfy is anadjustable voltage input to the control gate for this operation. In theconventional ETOX cell, Versvfy is set to be around +2.5V to reduce thenumber of over erased cells because the correction cannot be performedin a bit-by-bit mode as in regular program operation. The correction isdone in a collective mode. When the number of over-erased cellsincreases to some level, the correction current will overload theon-chip charge pump and fail to recover the Vt of the cells. Incontrast, Versvfy is set to be +1V in the present invention and as aresult of the 3-level word line voltage is used to perform bit-by-bitcorrection. There are many cells in many bit lines, but only one cellper one selected bit line is corrected simultaneously. Therefore,current over load will not occur and the corrected Vt can be setaccurately.

In FIG. 8 is shown a conventional ETOX NOR type flash EEPROM memoryarray 10. This nonvolatile NOR-type memory array includes: a matrix ofword lines and bit lines intersecting one another; and an ETOX memorycell being disposed so as to correspond to each intersection of thematrix of the global bit lines BLn-BLn+1, local bit line Bn, source lineSLn and global word lines WLn, the memory cell including a control gate,a drain, a source and a P-substrate as shown in FIG. 1. The controlgates are coupled to a corresponding one of the row wise word linesWLm(n), the drains are coupled to a corresponding one of the localcolumn wise bit line Bn and one of the global bit line BLn selected bytransistor Tn gated by BT1 (n) and BT2(n), and the sources are coupledto a corresponding one of the local row wise source lines SLm. Thememory cell is capable of performing a FN erase and a CHE programoperation based on the three level word line of the present invention.The plurality of control signals of WLm(n), BT1 (n) and BT2(n)aregenerated from an X-decoder (word line decoder), local bit line decoder,local source line decoder, global bit line decoder and global sourceline decoder, respectively.

Shown in FIG. 9 are two Block Erase Operations for a conventional ETOXNOR type flash EEPROM memory array 10. One is edge erase operation, andthe other is a channel erase operation. For channel-erase operation, anerase voltage of −15V is coupled to the corresponding row wise wordlines, WL0(0)-WL511(0) for selected Block 0, and ground is coupled tothe non-selected word lines in the rest of the blocks. The drains of thecells are coupled to 0V by a corresponding the local column wise firstlevel metal bit lines B0-B3 and the second global metal bit linesBLn-BLn+1 via transistors of T0 and T1 which are gated by applying Vddto BT1 (n) and BT2(n) signals. The sources are coupled to 0V by acorresponding plurality of row wise source lines SL0(0)-SLm(0). Thechannel erase conditions are applied so that the memory cells of Ma-Mlin the word lines in Block 0 are capable of performing a FN eraseoperation. The Vt of the cells are decreased after the erase operationis performed. The flash cells of Mm-Mx in non-selected Block n are keptundisturbed. The erase operation is designed to be an iterative process.Each erase pulse width can be set to around 1 ms. Given a shorter erasepulse, a larger number of erase pulses are required.

In FIG. 10 is illustrated a Block Erase Verify operation with preferredvoltages for WLm(n), global BLn, global SLm, BT1(n), BT2(n). AssumingWL0(0) is firstly selected for Block erase verify, then Versvfy iscoupled to WL0(0). The rest of the word lines of WL1(0) to WL511(0) arecoupled to −4V to shut off any potential leakage caused by over-erasedcells that might exist in Block 0. The word line voltage of −4V is not afixed number but is set to be able to shut off any leakage currentresulted from cells with a Vt less than −4V. All source voltages SLm(n)are coupled to ground. Bit line BLn is coupled to 0V and BLn+1 iscoupled to 1V for the verify operation. Bit line BLn+1is thenselectively connected to a corresponding sense amplifier forverification. In the conventional ETOX array a total of eight senseamplifiers are needed for byte read and 16 sense amplifiers for wordread. The verification for the rest of the cells on the same word lineswill be controlled by connecting the sense amplifiers to the next groupof 8 bit lines. The process will be continued in the same page until allcells in the page are verified. Then the verify process is moved to thenext page of WL1(0) in Block 0. After five hundred and twelve word linesare verified, the block erase verify is terminated. With the successfulverification of block erase all cells in the selected block (Block 0)become a logical “0”. The data of the cells in the non-selected blocksremain the same without changes. Three word line voltages are usesconcurrently, which include Versvfy for the selected word line, −4V fornon-selected word lines in the selected block and 0V for non-selectedword lines in non-selected blocks. Although the cells in Block 0 are ata logical “0”, there could be over erase cells. The definition of anover erased cell in the present invention is a cell Vt ranging from anegative Vt to a positive Vt but below 0.5V. The over-erase cells willinduce leakage and result in false readings so that a Vt correctionoperation is required.

FIG. 11 shows an erase inhibit operation with preferred voltages forWLm(n), global BLn, global SLm, BT1 (n), and BT2(n). This operation isperformed on sub-block basis and is intended to set those sub-blocksthat have been successfully erased into a de-selected mode to preventfurther erase. For example, WL0(0) and WL1(0) are verified to have asuccessful erase and are set to be in Erase Inhibit mode to avoid thefurther erase pluses. The way to set erase inhibit is to set the wordline voltage from −15V (erase) to 0V (inhibit) for channel erase orsource voltage from +5V (erase) to 0V (inhibit) for an edge eraseoperation. In the erase inhibit mode the tunneling electric field isreduced so that no tunneling effect will take place. This operation doesnot require three concurrent word line voltages. In FIG. 12 a correctionoperation is illustrated with preferred voltages for WLm(n), global BLn,global SLm, BT1 (n) and BT2(n). The correction operation is performed ona bit by bit basis and is intended to correct those over erased cells toa Vt voltage that is positive but below +0.5V. The process is repeatedto correct all cells in one selected word line WL0(0) and then moved tocorrect the cells in next word lines of Block 0. A Vcorrection voltageis coupled to the first selected word line WL0(0) along with a bit linevoltage of 5V in order to perform a CHE soft program. The Vcorrectionvoltage is set to be less than +10V while the rest of the word lines ofWL1(0) to WL511(0) are coupled to −4V to shut off any potential leakagedue to over erased cells that might be existing in Block 0. The −4V isan approximate value and is of sufficient magnitude to be able to shutoff any leakage current resulting from cells with Vt less than −4V. Allsource voltages of SL0(0)-SLm(n) are coupled to ground along with bitline BLn. Bit line BLn+1 is couple to 5V for the correction operation.The process is continued in the same page until all cells in the pageare corrected and then the process moves to correct next page WL0(1) inBlock 0. After all the word lines in a block are successfully corrected,the correction process is terminated. With the successful correction,all cells in the selected block (Block 0) become “0”. The data of cellsin the non-selected blocks remain the same without changes. Thiscorrection operation uses three concurrent word line voltages,Vcorrection, −4V and 0V.

FIG. 13 illustrates a correction verify operation with preferredvoltages for WLm(n), global BLn, global SLm, BT1 (n), and BT2(n). Thisoperation is performed on bit-by-bit basis. In the present invention,this operation is intended to verify that those over erased cells arecorrected to Vt within +0.5V but below +1V. A voltage Vcorvfy is coupledto word line WL0(0) and the rest of the word lines, WL1(0) to WL511(0),are coupled to −4V to shut off any potential leakage caused by overerased cells that might be existing in Block 0. The voltage of −4V is anapproximate value and is set to be able to shut off any leakage currentwhich results from cells with Vt less than −4V. All source voltages ofSL0(0)-SLm(n) are coupled to ground along with the bit line BLn. The bitline BLn+1 is coupled to +1V for the verify operation. The process willbe continued in the same page until all cells in that page are correctedand verified. Then the operation moves to verify next page of WLn(0) inBlock 0. After all word lines in the block are successfully correctedand verified, the verify process is terminated. Three word line voltagesare used, which include Vcorvfy for the selected word line, −4V fornon-selected word lines in the selected block and 0V for non-selectedword lines in non-selected blocks.

In FIG. 14 is illustrated a CHE program operation with preferredvoltages for WLm(n), global BLn, global SLm, BT1 (n), and BT2(n). Thisoperation is performed after correction and is on a bit-by-bit basis.The process is continued until all cells are fully verified. A voltageVpgm, which is approximately 10V, is coupled to WL0(0), and theremainder of the word lines of WL1(0) to WL511(0) are coupled to 0V.Because all over erased cells have previously been corrected, the −4Vused to shut off any leakage is no longer needed. The CHE programoperation performs on a bit-by-bit basis and terminates when all cellsin same byte/word are programmed, coupled to a high Vt (>4V). Threeconcurrent word line voltages are not required for this operation.

In FIG. 15 a CHE program verify operation is illustrated with preferredvoltages for WLm(n), global BLn, global SLm, BT1 (n), and BT2(n). Thisoperation is performed in a similar manner as correction verify. Theonly difference is that the verify voltage for the programmed cell,Vpgmvfy, is set to be approximately +4V for cell Vt of +4V after a CHEprogram operation. Three concurrent word line voltages are not requiredfor this operation.

In summary, there are three preferred operations for an ETOX array thatrequire a three level voltage word line. These operations include blockerase verify, correction and correction verify. In the block eraseoperation, the word line of the selected page within the selected blockis couple with Versvfy, the word lines in the non-selected pages withinthe selected block are couple to a voltage approximately −4V, and theword lines in the non-selected blocks are couple to 0V. In thecorrection operation the word line in the selected page of the selectedblock is coupled to Vcorrection, the non-selected pages in the selectedblock are coupled to a voltage approximately −4V, and the word lines inthe non-selected blocks are couple to 0V. In the correction verifyoperation the selected page within the selected block is coupled withVcorvfy, the word lines in the non-selected pages within the selectedblock are couple to a voltage approximately −4V, and the word lines inthe non-selected blocks are couple to 0V.

The second embodiment of the present invention will be described withreference to FIG. 16 through FIG. 27 for an AND array on P-substrate byemploying the same three voltage word line technique of the presentinvention for some preferred operations. The erase operations use bothedge and channel to perform the FN method. Erase operation is carriedout to decrease Vt of the cells and program increases Vt. The currentusing FN program and FN erase only causes 10 pA and 10 nA, respectively,per cell.

In FIG. 16a is shown a simplified form of a flash cell on P-substrate ofthe present invention with three nodes of D, G and S. Where D, G and Sstand for drain, gate and source, respectively. The potential ofP-substrate is held to ground level and is not shown in the figure. InFIG. 16b a flash cell of the present invention shown biased for twotypes of erase operations with first set of voltages for D, G and Scoupled with +5V, −10V and 5V, respectively, for an edge erase, and thesecond set of voltages for D, G and S coupled with 0V, −15V and 0V,respectively, for channel erase. Where −10V and −15V are exemplaryvalues. The exact value and time of the gate voltage in the eraseoperation varies with different flash technologies. Gate voltages of−15V/−10V and source and drain voltages of 0V/+5V will result in FNtunneling in the channel region for channel erase and edge region foredge erase. The tunneling electrons will flow from the floating gate tothe P-substrate and the source and drain to decrease Vt of the cellafter a predetermined erase time. The erase operation can be performedon the basis of single page (word line), block (more than 2 word lines),multiple blocks (more than two blocks) and chip (all blocks).

In FIG. 16c is shown a flash cell of the present invention biased in twotypes of erase inhibit operations with D, G and S coupled to a first setof voltages +5V, +5V and 5V, respectively, for an, edge erase, and D, Gand S coupled to a second set of voltages 0V, 0V and 0V, respectively,for a channel erase. The erase inhibit operation can be performed on thebasis of single page (word line), block (more than 2 word lines),multiple blocks (more than two blocks) and chip (all blocks).

FIG. 16d shows a simplified form of a flash cell on a P-substrate of thepresent invention with the three transistor nodes of D, G and S biasedwith 0V, Vcorrection and 0V, respectively, in correction mode. Thecorrection operation is performed on a page basis and is intended toverify the Vt of all cells in one selected word line after an eraseoperation.

Referring to FIG. 16e, a simplified form of a flash cell is shown on aP-substrate of the present invention with three nodes of the celltransistor D, G and S biased with 0V, Vpgm and 0V, respectively, in aprogram operation. This operation is performed on page basis and isintended to simultaneously program the selected cells to high Vt (>4V).

FIG. 17a shows a simplified form of a flash cell on a P-substrate of thepresent invention with three nodes of the cell transistor D, G and Sbiased with +5V, Vpgm/Vcorrection and +5V, respectively, for program andcorrection inhibit operation. This operation is intended to preventnon-selected cells from programming and correction in the same word lineof the selected block.

In FIG. 17b a simplified form of a flash cell is shown on a P-substrateof the present invention with three nodes of D, G and S biased with+5V/0V, +2.5V and +5V/0V, respectively, for program operation and 0V,+2.5V, and 0V, respectively, for correction inhibit. This is intended toprevent non-selected cells on word lines not selected in the selectedblock from bit line disturb of programming and correction.

In FIG. 17c a flash cell of the present invention is shown biased in aprogram verify operation with nodes D, G and S coupled with +1V, Vpgmvfyand 0V. This operation can be performed on page basis. FIG. 17d shows aflash cell of the present invention biased in correction verifyoperation with D, G and S coupled with +1V, Vcorvfy and +0V,respectively. This operation can also be performed on page basis. InFIG. 17e a flash cell of the present invention is biased in a readoperation with nodes D, G and S coupled with +1V, Vread and 0V,respectively. This operation can be performed on page basis. FIG. 17fshows a flash cell of the present invention biased in an erase verifyoperation with nodes D, G and S coupled with +1V, Versvfy and +0V,respectively. This operation can be performed, on page basis.

In FIG. 18 is shown a conventional AND memory cell, NOR-type flashEEPROM memory array 20. This nonvolatile NOR-type memory array includesa matrix of word lines and bit lines intersecting one another. The ANDmemory cell Ma to Mx being disposed so as to correspond to eachintersection of the matrix of the global bit lines BLm to BLm+3, localbit line Bn, source line SLn and global word lines WLn. The memory cellincluding a control gate, a drain, a source and a P-substrate is shownin FIG. 1. The control gate is coupled to a corresponding one of therow-wise word lines WLm(n). The drains are coupled to a correspondingone of the local column-wise bit line Bn, and one of the global bitlines BLn is selected by transistor Tn gated by BT1(n). The sources Snare coupled to a corresponding one of the local row-wise source lineSL(n) via transistor Tn gated by ST(n), and the memory cell is capableof performing an FN erase and FN program operations based on the threelevel word line bias of the present invention. The plurality of controlsignals of WLm(n), BT1 (n), ST1 (n) are generated from X-decoder (wordline decoder), local bit line decoder, local source line decoder, globalbit line decoder and global source line decoder, respectively.

FIG. 19 shows an AND flash array biased in Random page Erase Operationwith selected page WL0(0) coupled with −10V and BLm to BLm+3 coupledwith +5V for edge erase. Word line WL0(0) is coupled with −15V andBLm-BLm+3 coupled 0V for channel-erase. Word lines WL 1 (0) throughWL31(0) are coupled to +5V for the edge erase operation and coupled to0V for the channel erase to reduce the erase disturb to thosenon-selected cells in the selected block, Block 0. The non-selectedcells include cells that are in cells Ma-Ml of Block 0. The nodes of therest of non-selected cells in the non-selected blocks are all biasedwith 0V for the drains, gates and sources. The bias condition for thenon selected cells prevents erase disturb of cells of Mm-Mx innon-selected blocks. After the Random page erase operation is performed,the Vt of cells in WL0(0) will be decreased and data “1” is stored inthe cells.

FIG. 20 shows a random page erase verify operation with preferredvoltages for WLm(n), global BLm, global SL(n), BT1(n) and ST1(n). Thisoperation is performed after the completion of a random page erase. Inthe present invention, this operation is intended to verify the Vt ofthose cells erased by verifying that the value of Vt is below +1V. Thisoperation can be carried out on page basis using a plurality of senseamplifies connected to BLm. Random page is an arbitrary page of anyblock selected to perform erase operation. A confirmed success by pageverify means all cells in the selected page have been erased to be data“0” with Vt below +1V. Some fast cells may have been over erased with Vtbecoming negative which requires a Vt correction (recovery) in thesubsequent operation. In this operation, Versvfy of +1V is coupled tothe selected word line (page) for data verification. The rest of wordlines are coupled to ground level to shut off the bit line leakage.Since only one word line is selected in this operation, no three voltageword line bias is required.

In FIG. 21 a block erase operation is shown with preferred voltages forWLm(n), global BLm, global SL(n), BT1 (n) and ST1(n). In the presentinvention, the block erase operation is intended to erase a plurality ofcells in a selected block simultaneously. A typical flash blockcomprises of thirty two word lines and thousands of bit lines. In FIG.21, cells in Block 0 are selected for erase. Part of the cells in Block0 include Ma, Mb, Mc, Md on word line WL0(0), Me, Mf, Mg, Mh on WL 1(0)and Mi, Mj, Mk and Ml on WL31(0). All word lines from WL0(0) to WL31(0)in Block 0 are coupled to −15V with all selected bit lines from BLm toBLm+3 coupled to 0V for channel erase. All word lines of WL0(0)-WL31(0)in Block 0 are coupled to −10V with all selected bit lines of BLm toBLm+3 coupled with +5V for edge erase. In both cases, source line SL(0)is held at ground level during the erase operation. In edge erase, BT1is coupled to +10V to transfer +5V from global bit lines of BLm throughBLm+3 to local bit lines of B0 through B3. The source control line ST1is coupled to 0V to shut off the local source lines of S0 through S3 tothe common source line of SL(0) in Block 0. In channel erase, BT1 iscoupled to Vdd to transfer 0V from global bit lines of BLm through BLm+3to local bit lines of B0 through B3. The source control line ST1 iscoupled to Vdd to connect the local source lines of S0 through S3 to thecommon source line of SL(0) in Block 0. Some fast cells may have beenover-erased with negative Vt that require a Vt correction (recovery) inthe subsequent operation. In this operation, three voltage are notrequired for the word lines. For the remaining word lines innon-selected blocks are all coupled to ground to avoid any erasedisturbance.

FIG. 22 shows a block erase verify operation with preferred voltages forWLm(n), global BLm, global SL(n), BT1 (n) and ST1(n). In the presentinvention, the block erase verify operation is intended tosimultaneously verify a plurality of cells in a selected block on pagebasis. Each of the thirty two word lines in Block 0 is sequentiallyselected for data verification. The cells in WL0(0) are selected at thesame time for page verify. In this operation; word line WL0(0) iscoupled to Versvfy voltage at approximately around +1V. The rest of wordlines in Block 0, WL1(0) through WL31(0), are coupled with −4V to shutoff leakage and avoid false verification. The −4V is used with theassumption that the Vt of all cells is not more negative than −4V afterblock erase operation. The non-selected cells in non-selected word linesin non-selected blocks, Block 1 through Block n, are grounded. In thismanner, three voltages are used for word lines to achieve a bit-by-bitverify in the present invention. The three word line voltages includeVersvfy for the selected word line, −4V for non-selected word lines inthe selected block and 0V for non-selected word lines in non-selectedblocks. When all cells in WL0(0) have been verified, the operation iscontinued to verify the next word line WL1(0). WL0(0) is switched to −4Vand WL1(0) is coupled to Versvfy. Then, the same steps used to verifyWL0(0) is repeated on WL1(0). When WL31(0) has been verified, theprocess will terminate. After the completion of this operation, allcells in Block 0 are “0” with Vt less than +1V.

Referring to FIG. 23, a block erase inhibit operation is shown withpreferred voltages for WLm(n), global BLm, global SL(n), BT1(n) andST1(n). The bias conditions for the block erase inhibit operation isprovided to further reduce the erase disturbance to those pages whichhave passed the erase and erase verify during the iterative eraseoperation. For example, except for word line WL31(0), word lines WL0(0)through WL30(0) have passed the erase verify. Word line WL31(0) iscoupled with erase pulse of −15V and −10V for the channel and edge eraseoperations, respectively. Word lines WL0(0) through WL30(0) are biasedto the preferred voltages of 0V for channel erase, or +5V for edgeerase, to reduce the disturb in the channel and edge erase operationsrespectively. The reduction in erase disturb is because the voltage dropacross gate-drain and gate-source has been reduced from 5V to 0V forthose cells such as Ma-Mh shown in FIG. 23. The three concurrent wordline voltages include 5V/0V that have been erase verified in Block 0,−10V/−15V on word line WL31(0) and 0v/0V applied to non-selected wordlines in non-selected blocks Block 1 through Block n, where the voltagedesignation is for edge erase/channel erase.

In FIG. 24 is shown a correction operation with preferred voltages forWLm(n), global BLm, global SL(n), BT1 (n) and ST1 (n). This operation isprovided to correct those pages with over-erased cells and can beperformed on bit-by-bit basis. For example, cells of Mb and Mc in wordline WL0(0) have been verified to be in an over-erase state requiring aVt correction. The over-erase state is defined as the Vt below +0.5V. Bycontrast, cells of Ma and Md have been erased successfully with Vtmeeting the desired value which is to be above +0.5V but below 1.0V. Thecorresponding bit lines of BLm and BLm+3 are coupled to +5V with BLm+1and BLm+2 coupled to ground to bias Ma and Md in erase inhibit statewith Mb and Mc in erase state. The rest of the word lines of WL1(0)through WL31(0) are applied with +2.5V to reduce the correction disturbin selected bit lines. Once WL0(0) is corrected successfully, WL0(0)will be switched to +2.5V and the operation is continued to the nextword line WL1(0). This operation will be continued to the last page onword line WL31(0). After the completion of the correction operation, allcells in Block 0 are “0” with Vt below +1V but above +0.5V. The threeconcurrent word line voltages required for this operation areVcorrection for WL0(0) in Block 0, +2.5V for word lines WL1(0) throughWL31(0) in Block 0, and 0V on non-selected word lines in non-selectedblocks Block 1 through Block n.

FIG. 25 shows a correction verify operation with preferred voltages forWLm(n), global BLm, global SL(n), BT1(n) and ST1 (n). This operation isprovided to verify that pages with cells that have been corrected have aVt below +1V and above +0.5V. For example, cells in WL0(0) can beverified collectively. The correction verify is same as a page readoperation. Therefore, through transistors T0-T3, all local bit lines Bnare connected to global bit lines BLm and sense amplifiers with a biasvoltage around +1V. All source lines are held to ground by connectinglocal source lines Sn to SL(0) by means of transistors T4 through T7gated by ST1(0). As shown in FIG. 25, three preferred voltages areapplied to all word lines concurrently. These three voltages includeVcorvfy on the selected word line, WL0(0) for data verification, −4V onWL1(0)-WL31(0) for shutting off bit line leakage and 0V on non-selectedword lines in non-selected blocks Block 1 through Block n for avoidingany undesired disturbance.

FIG. 26 shows a random page program operation with preferred voltagesfor WLm(n), global BLm, global SL(n), BT1 (n) and ST1(n). This operationis intended to change cell data of “0” to “1” on a page basis. The Vt ofthe cells is changed from +1.0V to more than +4V. In order to changecell data from “0” to “1” by using the cell bias conditions shown inFIG. 17a. In concert with Vpgm on word line WL0(0), the correspondingbit lines of BLm and BLm+1 are coupled to +5V to inhibit programming onMa and Mb cells, and BLm+2 and BLm+3 are coupled to ground to enableprogramming on Mc and Md cells. The rest of the word lines of WL1(0)through WL31(0) are coupled with +2.5V to reduce the +5V disturb inselected bit lines. In this example, the data of Ma and Mb is kept to“0” but the data of Mc and Md is changed to “1.” A preferred threevoltages are required for this program operation. These three voltagesinclude Vpgm on the selected word line, WL0(0) for channel program,+2.5V on WL 1(0)-WL31(0) for disturb reduction and 0V on non-selectedword lines in non-selected blocks Block 1 through Block n for zeroprogram disturb. Vpgm is approximately +15V in this operation.

Referring to FIG. 27, a random page program verify operation is shownwith the preferred voltages for WLm(n), global BLm, global SL(n), BT1(n)and ST1 (n). This operation is provided to verify those programmed cellsin the selected page in the selected block. For example, cells in WL0(0)can be verified collectively. The random page program verify is same aspage correction verify operation. The only difference is that Vpgmvfy islarger than Vcorvfy used in the correction verify operation. Vpgmvfy isusually set to be around +4V for checking the Vt of programmed cells.This operation does not require three concurrent word line voltages.

The third embodiment of the present invention is explained withreference to FIG. 28 through FIG. 37. The cells are ETOX cells and areformed on a p-well. The p-well voltage is not always held to groundlevel as are the cells which are formed on a P-substrate. The detailedvoltages for drain, gate, source and p-well nodes are shown in FIG. 28athrough FIG. 29d. The operations which use the three voltage word lineof the present invention include block erase verify, erase inhibit,correction and correction verify. The operations of this embodiment aresimilar to the operations shown in FIG. 8-FIG. 15.

FIG. 28b shows a flash cell with bias conditions of the presentinvention illustrating an erase operation. The nodes D, G, S and Pw arecoupled with +5V, −10V, +5V and +5V, respectively, for channel erase,where voltage values are exemplary values. The exact value and time ofthe control gate's voltage is subject to different flash technologies.The gate voltage of −10V, and source and drain voltage of +5V willresult in a tunneling electric field in channel region of the cell. Thetunneling electric field will transport electrons from the floating gateto the P-well in order to decrease the Vt of the cells (off-state) aftera predetermined erase time. The erase operation can be performed on thebasis of single-page (word line), block (N word lines), multiple blocks(M blocks) or chip (all blocks), where N and M are typically larger thantwo.

FIG. 28c shows a flash cell with bias conditions of the presentinvention illustrating an erase inhibit operation. The first eraseinhibit is performed with D, G, S and Pw nodes set to +5V, +5V, +5V,+5V, respectively, and the second erase inhibit is performed with D, G,S and Pw nodes set to 5V, 5V/0V, 5V and 5V, respectively. This operationis intended to prevent a build-up of disturbance to those non-selectederased cells (in either selected or non-selected blocks) and to achievebetter endurance (number of program and erase cycles). Both gatevoltages, 0V and 5V, are optional inhibit voltage. The “0V” creates lessvoltage stress on WL decoder but more disturbances. The “5V” has morevoltage stress on WL decoder but less disturbances. The disturbance ofboth cases is too small to affect the value of Vt even during thespecified maximum erase time.

FIG. 28d shows a flash cell with bias conditions of the presentinvention that illustrates the correction operation with D, G, S and Pwnodes coupled to +5V, Vcorrection, 0V, and 0V, respectively. The exactvalue and time for the control gate voltage in this operation varieswith different flash technologies. The correction operation is a softprogram CHE operation. The difference between program and correctionlies in the control gate voltage. Normal program operation has about+10V applied to control gate and is intended to increase Vt to more than+5V. Correction has lower control gate voltage to avoid over program. Itis used to correct the Vt of cells back to around +1V from eithernegative or below +0.5V to avoid sub-threshold leakage during subsequentread or program operations. The operation of FIG. 28d is sometimesreferred to as recovery. The data becomes a logical “0” after thisoperation.

FIG. 28e shows a flash ETOX cell with the bias conditions of the presentinvention illustrating the CHE program operation with D, G, S and Pwnodes coupled to +5V, Vpgm, 0V and 0V, respectively. In the CHE program,there is a conduction current flowing from drain to source and causingan electron-hole pair generated at drain node. Electrons are attractedto the floating gate to increase Vt by the positive high voltage Vpgm,which increases the Vt of the cells. The CHE program typically consumesmore than 300 uA per cell. With weak on chip charge pump circuitsoperating at Vdd below 3V, only about 4 bits can be programmedsimultaneously. The cell data becomes a logical “1” after this operationis performed.

In FIG. 29a, a flash cell with bias conditions of the present inventionillustrates the program verify operation with D, G, S and Pw nodescoupled to +1V, Vpgmvfy, 0V and 0V, respectively. The Vpgmvfy is anadjustable voltage input to the control gate of the cells to meetdifferent Vt requirements in the program operation. For example, forstorage of more than 2-bits per cell, Vpgmvfy may vary from as low as 3Vup to about 5V or more. For a binary program, Vpgmvfy is set to bearound +5V. For storage of multiple states such as 1V, 2V, 3V, and 4V,Vpgmvfy is set to 1V, 2V, 3V and 4V accordingly to verify each state

In FIG. 29b, a flash cell with bias conditions of the present inventionillustrates the correction verify operation with D, G, S and Pw nodescoupled to +1V, Vcorvfy, 0V and 0V, respectively. The Vcorvfy is anadjustable voltage input to the control gate to meet different Vtrequirements in this operation. The Vt of over erased cells will berecovered back to a Vt-window of between +0.5V and 1.0V, after thisoperation is performed.

FIG. 29c shows a flash cell with bias conditions of the presentinvention illustrating a read operation with D, G, S and Pw nodescoupled to +1V, Vread, 0V and 0V, respectively. The Vread is anadjustable voltage input for the control gate for the read operation.Vread can be simply set to Vdd; however, in some designs, Vread is setto a clamped value so that the read voltage applied to the control gatecan be independent of Vdd variation. In some designs, the Vread voltageis boosted to be higher than Vdd, resulting in higher read current.

Shown in FIG. 29d is a flash cell with bias conditions of the presentinvention illustrating an erase verify operation with D, G, S and Pwnodes coupled to +1V, Versvfy, 0V and 0V, respectively. The Versvfy isan adjustable voltage input to the control gate for this operation. Inthe conventional ETOX cell, Versvfy is set to be around +2.5V to reducethe number of over erased cells because the correction cannot beperformed in truly bit-by-bit mode as in regular program operation. Thecorrection is done in a collective mode. When the number of over-erasedcells increases to some level, the correction current will overload theon-chip charge pump and fail to recover the Vt of the cells. Incontrast, Versvfy is set to be +1V in the present invention and as aresult of the 3-level word line voltage is used to perform bit-by-bitcorrection. There are many cells in many bit lines, but only one cellper one selected bit line is corrected simultaneously. Therefore,current over load will not occur and the corrected Vt can be setaccurately.

In FIG. 30 is shown a conventional ETOX NOR type flash EEPROM memoryarray formed on a P-well 30. This nonvolatile NOR-type memory arrayincludes: a matrix of word lines and bit lines intersecting one another;and an ETOX memory cell being disposed so as to correspond to eachintersection of the matrix of the global bit lines BLn-BLn+1, local bitline Bn, source line SLn and global word lines WLn. The memory cellincluding a control gate; a drain, a source and a P-well is as shown inFIG. 28. The control gates are coupled to a corresponding one of the rowwise word lines WLm(n), the drains are coupled to a corresponding one ofthe local column wise bit line Bn and one of the global bit line BLnselected by transistor Tn gated by BT1 (n) and BT2(n), and the sourcesare coupled to a corresponding one of the local row wise source linesSLm The memory cell is capable of performing a FN erase and a CHEprogram operation based on the 3-level word line of the presentinvention. The plurality of control signals of WLm(n), BT1 (n) andBT2(n) are generated from an X-decoder (word line decoder), local bitline decoder, local source line decoder, global bit line decoder andglobal source line decoder, respectively.

Shown in FIG. 31 is a block erase operations for a conventional ETOX NORtype flash EEPROM memory array formed in a P-well 41 and 42. The blockerase is an channel erase operation. For a channel erase operation, anerase voltage of −10V is coupled to the corresponding row wise wordlines, WL0(0)-WL511(0) for selected Block 0, and ground is coupled tothe non-selected word lines in the rest of the blocks. The drains of thecells are left floating by the corresponding local column wise firstlevel metal bit lines B0-B3 and the second global metal bit linesBLn-BLn+1 via transistors of T0 and T1 which are gated by applying 0V toBT1 (n) and BT2(n) signals. The sources are coupled to 5V by acorresponding plurality of row wise source lines SL(0). The P-wellvoltage for Block 0 is set to +5V, and the P-well for the blocks notbeing erased, Block n is set to 0V. The channel erase conditions areapplied so that the memory cells of Ma-Ml in the word lines in Block 0are capable of performing a FN erase-operation. The Vt of the cells aredecreased after the erase operation is performed. The flash cells ofMm-Mx in non-selected Block n are kept undisturbed. The erase operationis designed to be an iterative process. Each erase pulse width can beset to around 1 ms. Given a shorter erase pulse, a larger number oferase pulses are required. Three concurrent word line voltages are notrequired in this block erase operation.

In FIG. 32 is illustrated a block erase verify operation for an ETOX NORarray formed in a P-well with preferred voltages for WLm(n), global BLn,global SL(n), BT1(n), and BT2(n). Assuming WL0(0) is first selected forBlock erase, then Versvfy is coupled to WL0(0). The rest of the wordlines of WL1(0) to WL511(0) are coupled to −4V to shut off any potentialleakage caused by over-erased cells that might exist in Block 0. Theword line voltage of −4v is not a fixed number but is set to be able toshut off any leakage current resulted from cells with a Vt less than−4V. All source voltages SL(n) are coupled to ground. Bit line BLn iscoupled to 0V and BLn+1 is coupled to 1V for the verify operation. Bitline BLn+1 is then selectively connected to a corresponding senseamplifier for verification. The P-wells 41 and 42 for all blocks arecoupled to 0V. The process is continued in the same page until all cellsin the page are verified. Then the verify process is moved to the nextpage of WL1(0) in Block 0. After five hundred and twelve word lines areverified, the block erase verify is terminated. With the successfulverification of block erase all cells in the selected block (Block 0)for erase operation become a logical “0”. The data for cells in thenon-selected blocks remain the same without changes. Although the cellsin Block 0 are at a logical “0”, there could be over erased cells. Thedefinition of an over erased cell in the present invention is a cell Vtranging from a negative Vt to a positive Vt but below 0.5V. Theover-erase cells will induce leakage and result in false readings sothat a Vt correction operation is required. The three concurrent wordline voltages, Versvfy, −4V and 0V are required for this operation.

FIG. 33 shows an erase inhibit operation for an ETOX NOR array formed ona P-well with preferred voltages for WLm(n), global BLn, global SL(n),BT1 (n), and BT2(n). This operation is performed on sub-block basis andis intended to set those sub-blocks that have been successfully erasedinto a de-selected mode to prevent further erase. For example, WL0(0)and WL1(0) are verified to have a successful erase and are set to be inErase Inhibit mode to avoid the further erase pluses. The way eraseinhibit is set, the word line voltage is set to +5V/0V for channel erasewith the drain floating and the source voltage at +5V. In the eraseinhibit mode the tunneling electric field is reduced so that notunneling effect will take place. The P-well voltage for the activeblock, Block 0, is +5V, and the P-well voltage for the other blocks nothaving an erase operation is 0V. Three concurrent word line voltages arenot required for this operation; however, if the operation is used inconjunction with an erase operation there would be three concurrent wordline voltages, −10V for word lines being erased, +5v for inhibiting wordlines in the selected block and 0v for word lines in non-selected blocksBoth gate voltages, 0V and 5V, are optional inhibit voltages. The“0V”causes less voltage stress on WL decoder but more disturbance. The“5V” causes more voltage stress on WL decoder but less disturbance. Thedisturbance of both cases is too small to affect the value of Vt evenduring spec maximum erase time.

In FIG. 34 a correction operation is illustrated with preferred voltagesfor WLm(n), global BLn, global SL(n), BT1 (n) and BT2(n). The correctionoperation is performed on a bit-by-bit basis and is intended to correctthose over erased cells to a Vt voltage that is above +0.5V but below+1.0V. The process is repeated to correct all cells in one selected wordline WL0(0) and then moved to correct the cells in next word lines ofBlock 0. A Vcorrection voltage is coupled to the first selected wordline WL0(0) along with a bit line voltage of 5V in order to perform aCHE soft program. The Vcorrection voltage is set to be less than +10Vwhile the rest of the word lines of WL1(0) to WL511(0) are coupled to−4V to shut off any potential leakage due to over erased cells thatmight be existing in Block 0. The −4V is an approximate value and is ofsufficient magnitude to be able to shut off any leakage currentresulting from cells with Vt less than −4V. All source voltages ofSL(0)-SL(n) are coupled to ground along with bit line BLn. Bit lineBLn+1 is couple to 5V for the correction operation. The P-well of allblocks is couple to 0V. The process is continued in a same page untilall cells in same page are corrected and then the process moves tocorrect next page WL0(1) in Block 0. After all word lines in a block aresuccessfully corrected, the correction process is terminated. With thesuccessful correction, all cells in the selected block (Block 0) become“0”. The data of cells in the non-selected blocks remain the samewithout changes. This operation requires the use of three concurrentword line voltages, Vcorrection, −4V, and 0V.

FIG. 35 illustrates a correction verify operation with preferredvoltages for WLm(n), global BLn, global, SL(n), BT1 (n), BT2(n). Thisoperation is performed on bit-by-bit basis. In the present invention,this operation is intended to verify that those over erased cells arecorrected to Vt within +0.5V but below +1V. A voltage Vcorvfy is coupledto word line WL0(0) and the rest of the word lines, WL1(0) to WL511(0),are coupled to −4V to shut off any potential leakage caused by overerased cells that might be existing in Block 0. The voltage of −4v is anapproximate value and is set to be able to shut off any leakage currentwhich results from cells with Vt less than −4V. All source voltages ofSL(0)-SL(n) are coupled to ground along with the bit line BLn. The bitline BLn+1 is coupled to +1V for the verify operation. The P-well of allblocks is coupled to 0V. The process is continued in the same page untilall cells in that page are corrected and verified. Then the operationmoves to verify next page of WLn(0) in Block 0. After all word lines inthe block are successfully corrected and verified, the verify process isterminated. This operation requires three concurrent word line voltages,Vcorvfy, −4V and 0V.

In FIG. 36 is illustrated a CHE program operation with preferredvoltages for WLm(n), global BLn, global SL(n), BT1 (n), and BT2(n). Thisoperation is performed after correction and is on a bit-by-bit basis.The process is continued until all cells are fully verified. A voltageVpgm, which is approximately 10V, is coupled to WL0(0), and theremainder of the word lines of WL1(0) to WL511(0) are coupled to 0V.Because all over erased cells have previously been corrected, the −4vused to shut off any leakage is no longer needed. The CHE programoperation continues on a bit-by-bit basis and terminates when all cellsin same byte/word that are programmed, coupled to a high Vt (>4V). ThisCHE program operation does not require three concurrent word linevoltages.

In FIG. 37 a CHE program verify operation is illustrated with preferredvoltages for WLm(n), global BLn, global SL(n), BT1 (n), and BT2(n). TheP-well voltage is set to 0V. This operation is performed in a similarmanner as program verify in FIG. 15. This program verify operation doesnot require three concurrent word line voltages.

The fourth embodiment of the present invention will be described withreference to flow charts of FIG. 38 to FIG. 44 to illustrate the threeconcurrent word line voltages technique of this invention. In FIG. 38ais shown the Vt distribution of ETOX cells of prior art for a largeblock after performing FN block erase and CHE block program operations.Prior to the erase operation, the data of the cells contained both “1”and “0” data of different Vt voltages. All cells are pre-programmed tohigh Vt state above Vt1 as shown in waveform 600. Typically, Vt1 isapproximately +4V for a binary-data cell. Since the pre-programoperation is performed on a bit-by-bit basis, the Vt distribution of the“1” for programmed cells can be controlled to be very narrow. The flashcells with Vt larger than Vt1 store “1” data.

Continuing to refer to FIG. 38a, subsequent to the pre-programming, anFN erase operation is performed on a block basis to lower Vt of allprogrammed cells. This operation is to change the cell data from “1” to“0” in the selected block and creates a distribution as seen in waveform500. After block erase, the Vt of the cells in the selected block arebrought lower and below Vt0. The cells with a Vt below Vt−1 are referredto as over-erased cells 501 that require a correction 502. In the NORtype flash array, the voltage Vt−1 is the lowest acceptable positive Vt(not negative) to guarantee there is no occurrence of the over eraseproblem 501 for normal operation. Since the erase operation is performedon block basis and is less controllable than the program operation doneon a bit-by-bit basis. As a result, the Vt distribution of “0” data 500after block erase operation is much broader than the “1” data. Thevoltage Vt−1 in FIG. 38a is approximately +0.6V, Vt0 is approximately+2.5V and Vt1 is approximately +4V for 3V operation.

Referring to FIG. 38b, a Vt distribution is shown of ETOX cells in alarge block when the three concurrent word line voltages of the presentinvention are used. As seen from waveforms 550 and 650, the Vtdistributions of the programmed state of data “1” and the block erasedstate of data “0” can be made very narrow. The detailed explanation toshow how to achieve the narrow-distribution of data “0” and data “1” iswith reference to the flow diagram of FIG. 39. Over erased cells 551 arecorrected 552 using a correction operation discussed with reference tothe flow diagram on FIG. 40.

FIG. 39 shows a simplified flow diagram of a block erase operation whenthe present invention is applied to an ETOX cell. The block eraseoperation starts with selecting one block 51 which sets up a pluralityof addresses for an X-decoder and Y-decoder to select the right block.This is then followed by a FN block erase operation 52 to decrease theVt of the selected cells in the selected block. The erase operation isan iterative process, and each time an erase pulse of a pre-determinedtime is performed on the selected block, all erased cells are verifiedto determine if the Vt value is below Vt0 53 on a sub-block basis. Anysub-blocks that have been verified to satisfy that Vt is below Vt0 areimmediately set to an inhibit state by sub-block erase inhibit 54 toavoid further erase operation. The process continues to check determineif all sub-blocks are verified 53. The process stops when all pages ofthe sub-block are verified successfully 55. Any sub-block that is notinhibited 56 is returned to the erase operation 52. In the process shownin the flow diagram of FIG. 39, three concurrent applied word linevoltages are used in some steps. The first is in decision step 53(Versvfy, see FIG. 10 and FIG. 32) and the second one is in step 54 seeFIG. 33).

FIG. 40 shows a preferred flow diagram for a correction operation of thepresent invention. This operation is intended to correct those overerase cells in an erased block. Any cells with Vt below Vt−1 must becorrected to a voltage above Vt−1. With three concurrent word linevoltages, this operation can be performed on a bit-by-bit basis. Itshould be noted that correction is performed byte by byte, butcorrection will depend on the threshold voltage of each individual bitin the byte. The purpose is to avoid any over-correction. Thus,“bit-by-bit” means that the correction inhibition will stop correctionon any successfully corrected bit. Therefore, a narrow Vt distributionof data “0” is achieved. The first step is to perform a check 57 is toverify that Vt<Vt−1. If the Vt of any cells is below Vt−1, the cells arecorrected 58 to increase the VT of those cells. Since step 57 is carriedout on bit-by-bit basis, all bytes in the selected block are checked 59.The next byte 60 is verified until all pages in selected block have beenverified to have a Vt>VT−1. Whenever an over erase byte is found, animmediate byte correction is performed 58 and Vt is increased above Vt−1for the over erased byte. The reason for byte-correction is that an ETOXcell which uses CHE correction which consumes too much power and can notbe performed on page basis. Each page contains a plurality of bytes, andthe byte correction operation is performed consecutively byte by byteuntil all bytes in the selected page are corrected. The correctionoperation uses three concurrent applied word line voltages, Vcorrectionfor byte correction 58 and Vcorvfy for byte correction verification 57.

FIG. 41 a shows the Vt distribution for AND cells in prior art of alarge block after a FN block erase and a FN page program operations onall pages in a block are performed. Prior to the erase operation, thedata of the cells contained both “1” and “0” at different Vts. All cellswere erased to high Vt state (above Vt1) as shown in waveform 300.Typically, Vt1 is around +4V for a binary-data cell. Since the eraseoperation is performed on block basis, the Vt distribution of “1” data300 of the erased cells can not be controlled well. As a result, a wideVt distribution of “1” data 300 is generated in prior art. Subsequent tothe erase operation, a FN page program operation is performed to lowerVt of all erased cells. This operation changes the cell data from “1” to“0” in the selected block as seen in waveform 200. After the pageprogram, the Vt of the cells is brought lower below Vt0. Cells with a Vtbelow Vt−1 are defined as over-erased cells 301 that require Vtcorrection. In a NOR type flash array, the voltage Vt−1 is the lowestacceptable positive Vt (not negative) in order to guarantee nooccurrence of the over erase problem for a normal operation. Since theprogram operation is performed on page basis, it is more controllablethan the erase operation which is performed on a block basis. As aresult, the Vt distribution of “0” data 200 is much narrower than thedistribution of “1” data 300. The voltage Vt−1 is approximately +0.6V,Vt0 approximately +1.0V and Vt1 is approximately +4V for a 3V operation.

In FIG. 41b is shown a Vt distribution for AND cells of second priorart. Unlike the approach in FIG. 41a, the program operation is toincreases the Vt of the cells 200 and is carried out on page basis forall pages in a block. The erase operation decreases the Vt of the cells300 and is carried out on block basis. Therefore, “1” data has narrowerVt distribution than “0” data because there is more control over theoperation carried out on a page basis. The Vt0 in FIG. 41b is set to bemuch higher than Vt0 in FIG. 41a, because the cell program is performedon page with respect to the prior art of FIG. 41a, and the eraseoperation in FIG. 41b is performed on block basis. The prior art of FIG.41a uses a bit-by-bit program scheme to obtain “0” data so thatover-erase will not occur. The Vt0 can be set around +1.0V with Vt−1 of+0.5V in FIG. 41a. The prior art of FIG. 41b uses block erase, and theerase operation cannot be performed on a bit-by-bit basis. In FIG. 41b,Vt0 is set at higher value +2.5V to reduce the number of over-erasedcells.

FIG. 41c shows the Vt distribution for cells of the present inventionand having a tight distribution for “1” data 250 and “0” data 350. Allcells in a selected block are first collectively erased below Vt0 by FNtunneling as shown in distribution 350. The voltage Vt0 is set to beapproximately +1.0V for the present invention. Any over erased cells 351with a Vt below Vt−1 (+0.5V) is corrected 352 back above Vt−1 but belowVt0 by means of the bit-by-bit correction operation of this invention.Vt0 is used for block erase verify and Vt−1 used for page correctionverify. Although erase is performed on block basis, the post-erase,bit-by-bit correction makes a very tight Vt distribution for “0” data350 in the present invention. A FN channel program operation isperformed to increase Vt of selected cells on a page basis for all pagesin a block. This operation changes the data of the cells from “0” to “1”in the selected block and results in the distribution of thresholdvoltages as seen in waveform 250. After the FN channel program, the Vtof the cells are raised above Vt1.

Continuing to refer to FIG. 41c, in a NOR-type flash array, the Vt−1voltage is the lowest acceptable positive Vt to guarantee no occurrenceof the over erase problem in a normal operation. Since the programoperation is performed on page basis for all pages in a block, it ismore controllable than the block erase operation. As a result, a tightVt distribution of “1” data 250 is achieved. By using the techniques ofthe present invention, both tight Vt distribution of “1” data 250 and“0” data 350 can be attained.

FIG. 42a shows a simplified flow diagram of a write operation for an ANDcell. The write operation includes a block erase 20 to increase the Vtof cells above Vt1 collectively and a page program 21 to selectivelydecrease the Vt below Vt0. FIG. 42b shows a simplified flow diagram of awrite operation for a second AND cell. The write operation includes ablock erase 22 to decrease Vt of the cells collectively below Vt0, andprogramming 23 selectively increase Vt above Vt1. In either FIG. 42a orFIG. 42b, block erase and program are based on concurrent two voltageword line designs, Vt0 and Vt1.

FIG. 43 shows a simplified flow diagram of a block erase operation foran AND-like cell of the present invention. This flow diagram starts byselecting a block 61 for erase. Then a block erase 62 is performed wherethe Vt of the cells in the block is decreased. After the first blockerase pulse is executed on all selected pages, a page verification 63 isperformed to check if Vt is below Vt0. If the selected page meets thecriteria Vt<Vt0, it will be set to the erase inhibit state 64 to preventfurther erasing; otherwise, the verification is continued to set anynext pages in erase inhibit. The page verification will stop when allpages 65 are verified and set to page erase inhibit. Subsequently, asecond erase pulse is selectively applied to those pages not in thestate of page erase inhibit. The operation is branched back to blockerase 62 where Vt is decreased when all pages are not in erase inhibit.Each time, the number of erased word lines will be reduced in the blockerase operation 62. The operation will terminate when all pages are setinto page erase inhibit. Unlike the method in FIG. 42a and FIG. 42b, thethree concurrent applied word line voltage technique of the presentinvention is used in the block erase operation shown in FIG. 43,verification of one page cells 63 (Versvfy), and page edge erase inhibit64 (+5V/0V, see FIG. 23).

FIG. 44 shows a preferred flow diagram for a correction operation of thepresent invention. This operation is intended to correct those overerase cells in the erased block. Any cells with Vt below Vt−1 (67) haveto be corrected to have a Vt, which is above Vt−1 using correction 68which increases the Vt of a cell. The page verification is continueduntil all pages 69 and 70 in a block have been corrected. With the threeconcurrent applied word line voltages, Vcorrection for page correction68 and Vcorvfy for page correction verification 67 this operation can beperformed on a bit-by-bit basis. The operation will stop when all pageshave been corrected.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for erasing a block of a flash memoryarray using three consecutive bias conditions, comprising: a) applying afirst set of bias conditions to a block of cells of a flash EEPROMmemory array to erase said block of cells, b) applying a second set ofbias conditions to a page of cells of said block of cells to verify thatcells in said page have a Vt (threshold voltage) less than apredetermined value, c) applying a third set of bias conditions toinhibit from further erasure said page of cells that are verified to beerased, d) continuing until all cells in said block are erased andverified to be erased.
 2. The method of claim 1, wherein applying saidfirst set of bias conditions includes a large magnitude negative voltageapplied to a word line to erase floating gates of said block of cells.3. The method of claim 1, wherein applying said second set of biasconditions includes a voltage applied to a word line to verify the Vt ofsaid page of cells is less than said predetermined value.
 4. The methodof claim 1, wherein applying said third set of bias conditions includesa voltage applied to a word line to inhibit cells on said word line fromfurther erasure.
 5. The method of claim 1, wherein said flash EEPROMmemory array is an ETOX NOR type memory array.
 6. The method of claim 5,wherein applying the second set of bias conditions causes a FN(Fowler-Nordheim) channel erase operation, or a FN edge erase operation.7. The method of claim 1, wherein said flash EEPROM memory array is aNOR type AND memory array.
 8. The method of claim 7, wherein applyingthe second set of bias conditions causes a FN channel erase operation.9. The method of claim 1, wherein blocks not being erased are biased toprevent an erase disturb.